Pneumococcal type 9V is one of the most common types causing pneumococcal disease in young children. Type 9V PS is contained in the current pneumococcal vaccine but it induces a low antibody response in young children. To increase its immune response the PS was conjugated to a protein carrier. A modified conjugation method was applied to link 9V PS covalently to inactivated pneumolysin. Type 9V PS was reacted with cystamine in the presence of carbodiimide. The amino group of the formed compound was reduced by dithiothreitol to form a thiol derivative. The carrier protein was reacted with N-succinimidyl bromoacetate to form bromoacetylated protein. The thiol derivative of PS was next reacted with bromoacetylated protein to form a PS-protein conjugate. This method has produced a 9V PS-protein conjugate with a stable covalent linkage. pregnant mice at 2 weeks of gestation and/or lactating mice at 1 week after delivery were injected with 9V PS-inactivated pneumolysin conjugate. Young mice at 2 weeks of age were given additional dose of conjugate. Two weeks after injection, young mice were challenged with 10 to 10 type 9V cells. Measurements of survival rate and bacterial clearance are in progress.